Exterior airframe joints are required to be flush to minimize aerodynamic drag. It is therefore desirable to prevent any portion of a mechanical fastener from protruding beyond a surface of an outer panel of a workpiece in which the fastener is installed. It is also preferred to prevent any portion of the fastener from being recessed within the workpiece or recessed as compared to the remainder of the fastener. Having all portions of the fastener and the surface of the outer panel of the workpiece in the same plane is referred to as the “flushness” of the fastener. Flushness goals are easily met with conventional fasteners such as solid rivets, lock bolts, and threaded pins with nuts. Many pull-type blind fasteners also exhibit acceptable installed flushness. However, threaded blind bolts have always suffered from the need to have a corebolt break-off location vary by as much as 0.103 inch so that, in some grip conditions, the corebolt either protrudes above the surface of the outer panel by this amount or, if the break-off location is positioned to never break above an exposed outer surface of an enlarged body head of the fastener, the corebolt breaks low, thereby leaving a cavity of up to 0.103 inch deep. In either case, the fastener oftentimes requires milling of the protruding corebolt or filling (potting) of the cavity to produce the desired level of aerodynamic flushness wherein the corebolt is substantially flush with the outer surface of the enlarged body head.
Protruding head fasteners are used in regions that are not aerodynamically critical and on the ground (i.e., non-flying equipment). It is also desirable with protruding head fasteners to limit protrusion of the corebolt above a top of the body head surface, so as to avoid snagging on clothing or causing an injury to persons.